1. Field of the Invention
This invention relates to a method of formation of a linear grating (diffraction grating) used for wavelength-division multiplexing and similar in optical communications.
2. Description of the Related Art
In order to transmit a plurality of optical signals with different wavelengths within a single optical fiber to increase the amount of information transmitted in communications using optical fibers, a diffraction grating to separate or to combine optical waves is used. In a linear grating, a silicon or other substrate with translucent properties is formed with the thickness in a stair-like structure, so that wave separation or combination is performed by changing the angle of diffraction of light for different wavelengths.
FIG. 2A to 2F of the accompanying drawings is a series of process diagrams showing an example of a conventional method of linear grating formation.
In this example, a silicon substrate 1 is used for the grating, and LSI manufacturing techniques are employed to form a seven-step linear grating. The manufacturing process will be described in detail below.
In Step 21 (FIG. 2A), first the surface of the silicon substrate 1 is divided into bands with a width of 7 μm, and each 7-μm band is divided into seven regions a through g with a width of 1 μm. Then, using photolithographic techniques, a resist pattern 2 is formed on the regions e, f and g among the seven regions. The resist pattern 2 is then used to etch and remove three steps' worth of material from the surface of the silicon substrate 1. Here, the height dimension of one step is, at minimum, approximately 0.2 μm.
In Step 22 (FIG. 2B), the resist pattern 2 is removed. As a result, the silicon substrate 1A, in which three steps' worth of the surface has been etched away in the regions a through d, is obtained, as shown in FIG. 2B.
In Step 23 (FIG. 2C), a resist pattern 3 is formed on the regions c, d, f and g of the surface of the silicon substrate 1A. Then etching is performed to remove two steps' worth of material from the surface of the silicon substrate 1A using the resist pattern 3.
In Step 24 (FIG. 2D), the resist pattern 3 is removed. As a result, as shown in FIG. 2D, five steps' worth of material are removed from the surface of the regions a and b, three steps' worth of material are removed from the surface of the regions c and d, and two steps' worth of material are removed from the surface of the region e, to obtain the etched silicon substrate 1B.
In Step 25 (FIG. 2E), the resist pattern 4 is formed on the surface regions b, d, e and g of the silicon substrate 1B. Then etching is performed to remove one step's worth of material from the surface of silicon substrate 1B using the resist pattern 4.
In Step 26 (FIG. 2F), the resist pattern 4 is removed. Thus, as shown in FIG. 2F, six steps' worth of material are removed from the surface region a, five steps' worth of material are removed from the surface region b, four steps' worth of material are removed from the surface region c, three steps' worth of material are removed from the surface region d, two steps' worth of material are removed from the surface region e, and one step's worth of material is removed from the surface region f by etching respectively, to obtain a silicon substrate 1C having a seven-step stair-shape surface.
Thereafter, the entire back surface of the silicon substrate 1C is etched to adjust the thickness appropriately, to obtain a seven-step linear grating.
Thus by using LSI manufacturing techniques, a small linear grating one edge of which is approximately 0.5 mm can be formed.
Although not linear gratings, similar techniques relating to methods for the formation of patterns in semiconductor substrates are described in, for example, three Japanese Patent Kokai (Laid-open Application) No. 6-252031, No. 10-254121, and No. 11-214280.
The conventional methods of forming the linear grating and other patterns have the following problems.
Referring to FIGS. 3A and 3B of the accompanying drawings, the problems with the conventional pattern formation methods are described.
For example, in Step 23 of FIG. 2C, suppose that the position of the resist pattern 3 formed on the surface of the silicon substrate 1A is shifted somewhat to the right from the intended position, as shown in FIG. 3A. As a result, some portions of the surface regions a and e of the silicon substrate 1A are covered by the resist pattern 3. When in this state the surface of the silicon substrate 1A is etched, and the resist pattern 3 is removed, then the portions of the regions a and e covered by the resist pattern 3 remain as protrusion defects X and Y on the silicon substrate 1B, as shown in FIG. 3B. Accordingly, there may be an impediment to subsequent manufacturing steps, and the completed linear grating may not have the desired characteristics.